@doomroar Or they might kill him for revenge and then deal with prison and the police later. Killing him in the stadium means almost certainly death, whereas escaping with him, killing him and then turning themselves to the police might just be what is needed.
Although right now they don't have evidence, if they kill him it will turn up in court, as they uncover the story behind Mahoroba with the help of Konatsu and the orders. They could earn a sympathetic sentencing and live to see freedom again.
(Of course that's not going to happen. Hibino flew somewhere and is rushing to find Ikuo with some kind of intel she gained. But there is some logic to what they're doing.)